1. Field of the Invention
This invention relates to a method of automatically controlling the position between two moving members at a desired set point in a mechanically contactless member by using an ultrasonic pulse.
2. Prior Art
Various systems of utilizing an ultrasonic wave as a measurement or communication means in water have heretofore been proposed, but there have been many difficulties in finding industrial application of such a proposal to a system of using an ultrasonic wave as a direct measurement means or as a transmission signal between a wave transmitter and a wave receiver in the air and controlling the position between two moving members by use of this measurement means or transmission signal in a mechanically contactless manner, because of the technical restrictions that will be described hereinafter. Namely, (1) When a wave transmitter and receiver are installed adjacent to a mechanical structure (almost all the cases virtually belong to this case), reflected waves overlap transmission waves to thereby make it difficult to transmit accurate ultrasonic pulse, because the area of transmission in which ultrasonic pulses are aerially transmitted is restricted by the mechanical structure. (2) When the wave transmitter and receiver are installed at a short distance from each other and in circumstances where there are many obstacles between them, standing waves tend to be produced and transmission of accurate pulse is difficult. (3) It is necessary for the wave transmitter and receiver to transmit and receive ultrasonic waves, respectively, while the transmitter and receiver are being moved, and trembling of air in accordance with such movement, or vibration coming from the fitting surfaces, or natural convection or the like is sensed as external noise and overlaps with the ultrasonic pulse that is transmitted through the air; accordingly, transmission of accurate ultrasonic pulse has been difficult.
Stated more particularly, FIG. 1B is an explanatory view schematically illustrating the ultrasonic pulse which the piezoelectric element of the conventional ultrasonic transmitter generates by one driving pulse. When a driving pulse is applied to the conventional wave transmitter through a pulse transformer, the piezoelectric element is heavily excited by a piezoelectric effect and sends out an ultrasonic pulse of a specified frequency (fo) into the air by a resonance characteristic. The ultrasonic pulse thus sent out shows heavy excitation when the driving pulse is applied to the piezoelectric element, but even after the driving pulse disappears, not only after-vibration continues for a long time, but also vibrations at the rear end face of the thickness vibrator spread through the case and come over to the front end face of the vibrator and leave reverberation and echo thereon slightly later than vibrations at the front end face of the vibrator, thus interferring in the spreading of the pulse. Accordingly, it is considered difficult in practical industrialization to send out an ultrasonic pulse corresponding to the driving pulse, excellent in damping characteristic and narrow in angle of direction. Namely, when a conventional transmitter sends out an ultrasonic pulse and a receiver having the same structure as the transmitter is installed in an opposed relation with the transmitter within a short distance therefrom and when there is an obstacle in the neighborhood of the transmitter and the receiver, the ultrasonic pulse sent out by the transmitter is received by the receiver and reflected by the obstacle and returned in part to the transmitter. At this time, if the transmitter is having after-vibrations, standing waves are generated by reflected waves and after-vibrations, and disturb ultrasonic transmission signals. It is known as an ultrasonic wave characteristic that an angle of direction is narrowed by a thickness vibrator. Therefore, according to the conventional casing method, the vibrations at the rear end face of the thickness vibrator spreading through the case slightly later than the vibrations at the front end face of the vibrator and come over to the front end face and leave reverberation and echo thereon, interfers in the transmission of ultrasonic wave and level down the angle of direction which the thickness vibrator originally has, and makes it difficult to send out a narrow directional ultrasonic pulse. The use of the conventional casing method is, therefore, difficult under the circumstance under which the area of transmission in which the ultrasonic pulse is transmitted is restricted by a mechanical structure. Alternatively, as a method which presently uses the ultrasonic wave between the wave transmitter and the receiver in the air is industrially using a method of detecting the interception of trembling of air or the interception of ultrasonic wave between the transmitter and the receiver. In order to obtain a gain in the transmitter and the receiver used in the method, namely, to make the maximum use of a piezoelectric effect, ultrasonic transmission large in capacity load is practiced by increasing the capacity of the piezoelectric element. On the other hand, because mechanical Q is reduced, such large capacity loaded transmission deters unnecessary radiation other than a limited frequency, and trembling of air responsive to movement, or vibration coming from the fitting surfaces, or natural convection or the like is sensed as external noise and laid over the ultrasonic transmission signals in use to thereby have rendered it difficult to make aerial transmission of an accurate pulse. For this reason, it has long been considered difficult in practical application to use an ultrasonic wave as a control means in the air.